(1) Field of the Invention
The invention relates to a method and a device for controlling the final thickness of a rolled product, at the exit of a tandem rolling mill, adapted to eliminate cyclic defects of variation of the thickness present in the product. In particular, the method eliminates the cyclic defects generated by the rolling mill stands, the defects of the rolls and their mounting in the bearings, the defects of false round and of non-circularity of the rolling rolls.
The invention applies in particular to cold tandem rolling mills for rolling metal strips, for example steel, but can be applied in general to any mill comprising several rolling stands operating in tandem for progressive reduction of the thickness of a product passing successively between the working rolls of said stands.
(2) Prior Art
It is known that a rolling mill comprises, in general, at least two working rolls mounted inside a support stand and defining an airgap for passage of product to be rolled, the stand bearing means for application of an adjustable clamping effort between the rolls. The number of rolls can vary according to the type of rolling mill, for example duo, quarto, sexto, according to whether it comprises a stack of two, four or six rolls to apply the clamping effort to the rolled product, or even other types of rolling mill.
These rolls are mounted to rotate in bearings known as chocks which may slide vertically inside the support stand to allow reclamping of said rolls and application of clamping effort to the product. Such an arrangement is well known and since it has been explained in numerous patents, it is not necessary to describe it further.
When mounting the rolls in these bearings presents defects due to the rolls or to their mounting in the bearings, or significant or irregular wear, concentricity defects, or even if the rolls themselves have a circularity defect, the result is a rotation of the rolls which is not perfectly circular, creating artificial and parasitic variation of the clamping effort.
To allow the product to pass between the rolls and a reduction in thickness, the said rolls are driven in rotation about their axis by motor means which apply a drive torque either directly to the working rolls, or indirectly to the support rolls or to the intermediary rolls in the case of quarto or sexto mounting.
Rolling mills known as <<in tandem”, comprising at least two successive stands each contributing to part of the reduction in thickness, have been known for some time now. From its raw thickness on entry to the first stand the product undergoes initial reduction in thickness in the first stand, and exits from there at a speed determined by the rotation speed of the working rolls of this stand. In the second stand it undergoes a second reduction in thickness and exits from there at a higher speed to respect the law of conservation of the mass flow rate. The working rolls of the second stand must be driven in rotation at a speed greater than that of the rolls of the first stand, the ratio of the speeds between the first and the second stand being in the inverse ratio of the reduction in thickness made by the first stand. This is how it happens from stand to stand according to the total number of rolling stands the tandem rolling mill comprises.
Also, the rotation torques applied to the working rolls are adjusted such that each stand exerts a traction on the strip leaving the preceding stand. To obtain the requested final thickness it is necessary to control on one hand the reduction of the thickness made in each of the rolling stands in order to get, at the exit of the mill, a product having a constant thickness with a certain degree of precision, and also to keep the strip perfectly tightened in each space known as “inter-stand” between two successive stands, so as to avoid reaching traction levels which would risk causing the strip to break.
Normally, monitoring the thickness of the strip as it passes through the successive stands of a tandem rolling mill is ensured by the monitoring of the mass flow rate, otherwise called “mass flow”.
Controlling the thickness of the tandem train is ensured so as to obtain a perfectly constant thickness of the strip at the exit of the mill, and for this it was imagined for a long time to maintain at a constant value on one hand the thickness of the strip at exit of the first stand, and on the other hand, the ratio of the speeds of the first and last stands.
The speeds of the intermediary stands can be deduced from these conditions as they are imposed by the law of conservation of the masses of metal passing through the stands of the rolling mill, and they are in the inverse ratio of the reductions to be attributed to each rolling stand. Controlling the thickness at the exit of the first stand is generally ensured, on a modern rolling mill, by a hydraulic clamping device which is controlled by a thickness gauge situated downstream of this stand. Certain more refined systems also comprise a thickness gauge upstream of this stand.
But systems for controlling thickness had been installed at a time when the clamping means of the rolls were constituted by electromechanical screw and bolt systems and adjusting was done by acting on the motors controlling rotation of the screws. Such tandem rolling mills have often been modernised by replacing the screw and bolt systems by hydraulic controls of which the position control precision and response time performances are superior. Yet mixed mills are still found, comprising, according to the stands, the two devices. All these clamping devices and their variants have been widely described and there is no need to do so further here.
The classic system for controlling of the thickness in the tandem rolling mill described earlier is currently called <<automatic gage control>> or AGC.
With this system it is thus not possible to act on the rotation speeds of the rolls of the stands to adjust the traction of the strip in the inter-stand intervals, without affecting the thickness at exit. The clamping devices of the stands are generally used to adjust the traction of the strip. Traction-measuring devices, generally constituted by an inter-stand rolling mill tensiometer, are installed for this purpose, which act to control the clamping means of the stand situated downstream. A thickness gauge placed at the exit of the rolling mill monitors the final thickness by acting on the speed of the last one or two stands of the tandem rolling mill. The inter-stand traction control system is also called <<automatic tension control>> or ATC.
Such controlling schemas would function perfectly with rolling stands which would present no mechanical defect, and in particular no defect in application of the clamping effort on the rolled product during rotation of the rolls. All these mechanical assemblies can have defects in mounting, adjustment, or even irregular wear which would create thickness defects in the product. In fact, these roll defects vary the clamping force on the product because, considering the variation in the value of their diameter throughout one rotation, the distance between the clamping force application means and the product is not constant. Everything happens as if adjusting the position of these clamping means were being changed, causing variation in the force applied, and due to this a variation in the thickness of the product. This thickness defect is cyclic and its frequency corresponds to that of the rotation of the roll. There would thus be relatively slow variations in thickness along the rolled product, corresponding to the eccentricity defects of the large-diameter support rolls, and faster variations corresponding to the circularity defects of the working rolls of smaller diameter.
Methods and devices for compensating for the effect on the thickness of the false round defects of the rolls were envisaged. These methods consist of determining a signal representative of the thickness defect caused by a support roll or a working roll. In general, processing by frequential analysis of the signal of the thickness gauge is carried out to extract variations corresponding to such or such roll of such stand, by tuning the frequency of the processing device to that of said stand. In more elaborate methods it was noticed that the false round signal of the rolls could also be extracted from that of the inter-stand traction measurement. French patent FR 2 735 046 to the applicant company accordingly proposes a method for compensation of the eccentricity defects of the rolls of a stand determined by generating a signal from measuring the traction in the strip upstream of this stand. Next, the compensation signal is used to correct adjustment of the clamping means of said stand.
Even though this method can be used to compensate the defects of all the rolls, it has been used primarily to compensate the defects of the support rolls. This is due to the fact that, considering their considerable diameter the rotation frequency is relatively low, of the order of a few hertz, to around 10 hertz. Now, these defects are detected by a thickness gauge of which the measurement is filtered. In fact, these gauges, generally by radiation, have a dispersion which translates by a background noise, the filtering of which is useful for making it easier to execute and adjust controlling. Typically, the filter of a thickness gauge is a low-pass filter of which the cutting frequency is regulated in the vicinity of 8 hertz. Because of this the defects due to the working rolls of which the diameter is around three times less that that of the support rolls remain unseen for most of the time at a high rolling speed.